When accessing WP-µ using administrator, or without being authentified, I have no problem at all. But, if you are authentified, and try to enter to another blog where you have no rights, it will try to reach wp-admin/ and it will fail, sending you to page http://wp-admin/ (or, if your DNS works like that, http://www.wp-admin.com).

It also happens if you are not authentified at all and try to enter in other blog of the multiblog where you have not any rights with your user. It means, you try to enter in http://myblog.com/mattsblog/wp-admin/ with the user of /jacksblog/ and it will reach http://wp-admin/